1. Field of the Invention
The present invention relates to a semiconductor laser device that operates stably and with high power, and particularly relates to a semiconductor laser device using a Group III-V nitride based semiconductor material.
2. Related Background Art
A semiconductor laser made of a Group III-V nitride based semiconductor material (AlxGayIn1-x-yN, where 0≦x≦1, 0≦y≦1) including gallium nitride is a key device for realizing ultra high-density recording using an optical disc, and is a violet semiconductor laser that is currently the nearest to a practical level. To increase the power of this violet semiconductor laser is a technology for allowing speedy writing with respect to an optical disc as well as a must technology for developing new technical fields such as the application to a laser display.
FIG. 7 schematically shows one example of a typical semiconductor laser in which a current confinement structure is formed with an insulation film. A stripe structure having a p-type conductive clad functions as a current confinement structure. Generally, a width of the stripe is uniform over the entire resonator. As a current injected from an electrode increases, a carrier density in an active layer increases, and when the value reaches a predetermined threshold carrier density, laser oscillation is attained. An optical output of the laser increases in proportion to the carrier density in the active layer. On the other hand, when the carrier density within the active layer is too high, a kink occurs due to spatial hole burning or a saturation of the optical output occurs due to a gain saturation, which impair the high optical output operation.
One of the effective measures for suppressing the generation of a kink is to narrow a width of a stripe. As a width of a ridge portion decreases, the distribution of carriers injected into an active layer and the expansion of an intensity distribution of light induced in the active layer in a transverse direction are narrowed relatively, thus suppressing the generation of a kink resulting from the spatial hole burning.
However, the narrowing of the width of the stripe uniformly over the entire resonator leads to an increase in series resistance of the device, which means an increase in driving voltage of the device. Especially, it is known that the reliability of a nitride based semiconductor laser critically depends on the driving voltage and a driving current, so that an increase in driving voltage should be suppressed to the minimum. Furthermore, a horizontal far-field of view, which is an important parameter of a semiconductor laser used for writing to an optical disc, also is determined by the width of the stripe. That is to say, the width of the ridge portion should be a value that can optimize each value of the current vs. optical output characteristics, the series resistance and the horizontal far-field of view of the device.
To cope with these challenges, JP 2000-357842 A discloses a laser structure in which a width of a stripe decreases toward both end faces of a resonator from a center portion of the resonator so as to form tapered regions. This structure can provide stable laser oscillation in a fundamental transverse mode without excessively increasing a driving voltage of the device, as compared with the conventional laser structure in which the width of the strip is narrowed uniformly.
In addition, “Semiconductor Laser” (written and edited by Kenichi IGA, published by Ohmsha, the 1st edition, Oct. 25, 1994, p. 238) describes, as an effective method for increasing an output of a semiconductor laser, to let the end faces of a resonator have asymmetry in reflectance. This is a general method in a semiconductor laser that is used for writing to an optical disc. According to this method, the end faces forming the resonator are coated with dielectric films so as to let the end faces have asymmetry in reflectance, where among the end faces forming the resonator, a front end face of the resonator from which principal laser light is emitted is made to have low reflectance, whereas a rear end face on the opposite side is made to have a high reflectance. For instance, the front end face has the reflectance of 10%, and the rear end face has 90%. The reflectance of a dielectric multilayer film can be controlled by a refractive index and a thickness of a dielectric layer used and the total number of the lamination.
However, in the case where the front end face and the rear end face making up the resonator have asymmetry in reflectance, a significant deviation occurs in the distribution of an optical intensity along an axis direction of the resonator within a semiconductor laser. FIG. 9 shows one example of the optical intensity distribution along the axis direction of the resonator in the conventional semiconductor laser shown in FIG. 7. As shown by curve A of FIG. 9, when the front end face and the rear end face both have the reflectance of 20%, the front end face and the rear end face are the same in optical intensity. On the other hand, as shown by curve B of FIG. 9, when the reflectance of the front end face is 10% and the reflectance of the rear end face is 90%, the optical intensity at the front end face is higher than the optical intensity at the rear end face by about two times.
On the other hand, in the conventional type laser structure shown in FIG. 7, in which the width of the stripe is constant over the entire resonator, a density of carriers injected into the active layer is uniform along the axis direction of the resonator as shown by curve C of FIG. 8. Thus, in the semiconductor laser to which coating is applied so as to make the front end face and the rear end face have asymmetry in reflectance, a situation occurs such that, while there is a significant deviation occurring in the optical intensity distribution between the front end face and the rear end face, the density of carriers injected into the active layer is uniform. That is to say, the carrier density in the active layer in the vicinity of the rear end face becomes excessive, which leads to a problem of the generation of a kink or gain saturation.
This phenomenon becomes remarkable especially in a nitride based semiconductor laser having an extremely high threshold carrier density and having a high differential gain, as compared with an infrared semiconductor laser made of an AlGaAs based semiconductor material (AlxGa1-xAs(0≦x≦1)) and an infrared semiconductor laser made of an AlGaInP based semiconductor material (AlxGayIn1-x-yP(0≦x≦1, 0≦y≦1)).
According to the nitride based semiconductor laser structure disclosed in JP 2000-357842 A, the stabilization of the fundamental transverse mode is sought by decreasing the width of the stripe toward the front end face and the rear end face of the resonator from the central portion of the resonator. However, in the case where the front end face and the rear end face of the resonator are significantly different in reflectance, the above-stated structure cannot correct the unbalanced state of the optical intensity distribution and the density distribution of the injected carriers between the front end face and the rear end face, so that it is difficult to avoid the formation of a region having an excessive density of carriers injected.